Onychopterella ( ON-ə-kop-tə-REL-ə, from Ancient Greek: ὄνῠξ (ónyx), "claw", and πτερόν (pteron), "wing") is a genus of predatory eurypterid ("sea scorpion"), an extinct group of aquatic arthropods. Fossils of Onychopterella have been discovered in deposits from the Late Ordovician to the Late Silurian. The genus contains three species: O. kokomoensis, the type species, from the Early Pridoli epoch of Indiana; O. pumilus, from the Early Llandovery epoch of Illinois, both from the United States; and O. augusti, from the Late Hirnantian to Early Rhuddanian stages of South Africa. Its prosoma (head) could be subquadrate (almost square) or subrectangular (almost rectangular), with reniform (kidney-shaped) eyes. The appendages (limbs) were generally long, narrow and pointed at their tip. The abdomen and telson (the posteriormost division of the body) had different shapes and sizes depending on the species. The ornamentation of the body consisted of small, pointed scales. The largest species of the genus was O. kokomoensis with a total length of 16 centimetres (6.3 inches) long, followed by O. augusti (14.3 cm, 5.6 in) and O. pumilus (4 cm, 1.6 in). The first Onychopterella fossils, belonging to O. kokomoensis, were discovered in 1896 at the Waterlime Group of Kokomo, Indiana. The species has received attention from eurypterid researchers for its terminal claw in the sixth pair of appendages or swimming legs. Onychopterella is also the type genus of the basal ("primitive") family of eurypterines Onychopterellidae together with Alkenopterus and Tylopterella, characterized by the presence of spines in the second to fourth pair of appendages and a lack of them in the fifth and sixth (except occasionally one on the distal end of the swimming leg), as well as the lanceolate (lance-shaped) or styliform (pen-shaped) form of the telson and other characteristics. The exceptional preservation of the fossils of O. augusti has permitted scientists to describe part of the alimentary canal that has only been observed in a few species of eurypterids, as well as the internal muscular structure of its limbs and even part of the external branchial respiratory system. This turned out to be highly similar to that of the scorpions of today, supporting a eurypterid-scorpion relationship. Onychopterella was a genus that was able to swim. Most of the time it was likely in the stratum, probably using its spines to walk and its head to dig in the ground.
Description
Like the other onychopterellids, Onychopterella was a small eurypterid. The smallest species, O. pumilus, measured only 4 centimetres (1.6 inches). The size of the largest one, O. kokomoensis, is estimated at 16 cm (6.3 in), representing the biggest species of the family Onychopterellidae. O. augusti had a similar size, with the largest specimen reaching 14.3 cm (5.6 in). The prosoma (head) had a subquadrate (almost square) shape in O. kokomoensis and O. augusti, while in O. pumilus it was subrectangular (almost rectangular). The outline of the carapace (the exoskeleton part covering the prosoma) varied according to the species. It was anteriorly rounded in the corners in O. kokomoensis, with almost straight lateral margins in O. pumilus, and resembled a horseshoe in O. augusti. In O. kokomoensis and O. pumilus, the eyes were reniform (kidney-shaped), standing out for their size and prominence in the latter species. The eyes of O. augusti are unknown. The metastoma (a large plate that is part of the abdomen) of O. kokomoensis was small, oval and slightly narrower anteriorly than posteriorly, while that of O. augusti was subcordate (almost heart-shaped) anteriorly and rounded posteriorly. The metastoma of O. pumilus is not known in its entirety. Only the posterior part, which was rounded, is known. In O. kokomoensis, the preabdomen (body segments 1 to 6) was as long as it was wide, while the postabdomen (segments 7 to 12) was short and compact and gradually increased in length posteriorly. It was the only species with considerably large epimera (lateral "extensions" of the segment) in the pretelson (segment that preceded the tail). In O. pumilus, the preabdomen was wider than long, the fourth segment being the widest. The postabodmen was longer, decreasing in width more rapidly in the eighth and ninth segments than in the rest. The length of the segments gradually increased towards the telson (the posteriormost division of the body). The opisthosoma (abdomen) of O. augusti is known in more detail; the preabdomen was a little wider than long, with raised areas representing branchial (of the gills) chambers or respiratory tissue of the branchial tract. The postabdomen was short, and its segments gradually narrowed towards the telson. Each of the postabdominal segments had small epimera. The shape of the telson in O. kokomoensis was clavate (resembling a club), in O. pumilus it was styliform (pen-shaped) and in O. augusti it was lanceolate (lance-shaped). The walking legs (second to fifth pair of appendages or limbs) were generally undifferentiated, long, narrow and with spines only at their distal end except in the fifth pair, which was not spiniferous. The swimming legs (sixth pair of appendages) were inconspicuous, very narrow and ending in a very long spike-like ninth podomere (leg segment). The swimming legs of O. augusti were easily recognizable from the other species by the prolongation of the eighth podomere into two lateral projections (parts of the body that protrude) and by the curved shape and great length of the spike-like podomere. The body ornamentation of Onychopterella, composed of small pointed scales, was the same in all species.
History of research
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